1. Field Of The Invention
This invention relates to a servosystem capable of preventing impartation of an abnormally high acceleration to an object to be controlled.
2. Description Of The Prior Art
As an example of applications of servosystem, there is a vibration tester which employs an electrohydraulic servo, for effecting a vibration tolerance test. In this vibration tester, a test structure which requires safety agaist severe vibrations is placed on a shake table which is a movable part of the vibration tester, and this table is vibrated by the associated electro-hydraulic servo, so as to subject the test structure to a vibration tolerance test. The vibration tester of the kind described above employing the electro-hydraulic servo for conducting the vibration tolerance test comprises generally a displacement control system. The basic structure and operation of this displacement control system will now be briefly described. A displacement reference input signal representing the desired displacement of the shake table is applied in the form of a voltage signal. This displacement reference input signal is compared in a comparator with a voltage signal obtained by converting the actual displacement of the shake table by a displacement detector. The comparator output signal indicative of the error between the desired displacement and the actual displacement, that is, the signal indicative of the controlled deviation, is applied to a servo amplifier to be subjected to current amplification, and the output signal of the servo amplifier is applied to a servo-valve. In response to this current input signal, the servo-valve controls the direction and flow rate of a hydraulic fluid supplied to a hydraulic cylinder driving the shake table until finally the controlled deviation between the desired displacement and the actual displacement of the shake table is nullified.
It is commonly acknowledged that, in such a servosystem, a high acceleration is produced in the shake table in the instant of emergency stop of the shake table. A device for suppressing such an acceleration is disclosed in Japanese Utility Model Publication No. SH051-42373 published Oct. 15, 1976. The displacement reference input signal is also another factor which imparts a high acceleration to the shake table. This displacement reference input signal is generally applied from a signal generator such as, a function generator, a data recorder or an electronic computer. In the event of occurrence of an abnormal operating condition in such a signal generator or in the event of application of an abnormal input signal due to a maloperation by the operator, an excessively high acceleration will be imparted to the shake table tending to destroy the test structure placed on the table. Also, when the applied input signal has a random waveform, it is difficult to predict the degree of acceleration produced in the shake table. However, when, for example, the test structure is manufactured at a great deal of costs, prevention of generation of an excessively high acceleration in the shake table is one of the important technical problems to be solved for the protection of the expensive test structure.